docs: def to_dict(self):
          """
          Returns a dictionary representation of a RegressionModel instance.
  
          """
          d = {
              'model_type': 'regression',
              'name': self.name,
              'fit_filters': self.fit_filters,
              'predict_filters': self.predict_filters,
              'model_expression': self.model_expression,
              'ytransform': YTRANSFORM_MAPPING[self.ytransform],
              'fitted': self.fitted,
              'fit_parameters': None,
              'fit_rsquared': None,
              'fit_rsquared_adj': None
          }
  
          if self.fitted:
              d['fit_parameters'] = yamlio.frame_to_yaml_safe(
                  self.fit_parameters)
              d['fit_rsquared'] = float(self.model_fit.rsquared)
              d['fit_rsquared_adj'] = float(self.model_fit.rsquared_adj)
  
          return d